1. Field of the Invention
Exemplary aspect of the present invention generally relate to a plastic optical element produced by molding, an optical scanner including the plastic optical element, and an image forming apparatus including the optical scanner.
2. Description of the Background Art
In order to achieve high-speed and high-quality imaging in color image forming apparatuses such as digital copiers and laser printers, four photoconductive drums, one for each of the colors yellow, magenta, cyan, and black, are arranged in tandem in a direction of conveyance of a sheet and exposed simultaneously by a plurality of light beams so that a latent image is formed on each of the photoconductive drums. The multiple latent images on the photoconductive drums are then developed with toner by developing devices of the respective colors, and transferred onto an intermediate transfer belt or a recording medium so that they are superimposed one atop the other.
Such image forming apparatuses employ an optical scanner for writing an image on the photoconductive drums. In general, the optical scanner is equipped with a plurality of light sources to project light beams for each color, a light deflector to deflect and scan the light beams, and a plurality of imaging optical systems including imaging optical elements for each light beam to focus the deflected light beams onto the photoconductive drums.
In such an optical scanner, as described above the imaging optical system is provided for each light beam, necessitating multiple imaging optical elements having the same shape. (For example, if the imaging optical system employs one imaging optical element per light beam, four imaging optical elements having the same shape are needed. In a case of using two imaging optical elements per optical system, eight optical elements are needed.) The drawback to this configuration is that the size of the optical scanner tends to be large, thereby defeating the purpose of reducing the size of the optical scanner.
In order to reduce the size of the optical scanner, in one approach, a plurality of the imaging optical elements of the imaging optical systems are laminated one atop the other and adhered together, and the imaging optical systems are disposed close to each other.
However, when laminating two imaging optical elements together, the optical elements need to be fixed together by some sort of adhesive, thus complicating manufacturing process. To address this difficulty, in order to reduce manufacturing steps and cost, the imaging optical elements are molded together by a molding process, forming an integral double-layer imaging optical element.
There is increasing demand for use of plastic material in the optical element employed in the optical scanner to reduce its cost. Furthermore, in order to provide the optical scanner with multiple capabilities, a mirror surface of the optical element tends to have a complicated aspheric shape. To fabricate such an optical element, molding is suitable for mass production at low cost.
More specifically, a mold assembly including a cavity having particular shapes associated with the functions of the optical element enables mass production of such an optical element at low cost. Forming an optical element using a resin injection molding process involves cooling the molten resin in the cavity inside the mold to harden the resin. In order to reduce deformation of the molten resin in the cavity, the internal pressure of the resin in the cavity is reduced intentionally.
However, there is a drawback to this approach in that, when the pressure of the molten resin in the cavity is reduced, the resin shrinks undesirably in the subsequent cooling and hardening process. Its shrinkage pulls the resin from the inner mold surface, thereby separating the resin from the mold surface and hence generating a sink in an optical surface of the optical element. A sink is a known phenomenon in the molding process in which a surface of the molten resin subsides.
In view of the above, a non-transfer portion on which the surface of the mold is not transferred, that is, a sink, is formed in one of non-optical surfaces of the optical element other than the optical surface. The non-optical surface is a surface through which no light passes. The sink is deliberately and preferentially generated in the non-optical surface, thereby preventing the sink from appearing undesirably in the optical surface when the pressure of the molten resin in the cavity is reduced.
More specifically, a single cavity piece that forms a surface other than the optical surface to the resin is slidably disposed. When the resin is cooled below its softening temperature, the slidable cavity piece is slid away from the resin, thereby forcibly forming a void between the resin and the cavity piece. Accordingly, the non-transfer portion (the sink) is formed in the resin. In this configuration, the non-transfer portion shrinks, deliberately forming the sink in the non-optical surface. As a result, the sink is prevented from appearing in the optical surface.
Alternatively, the cavity piece that forms the surface other than the optical surface includes a vent hole through which compressed air is blown so that the sink is generated deliberately in the surface contacting the compressed air, as the resin is cooled below its softening temperature. This method is advantageous for a relatively thin plastic optical element in a light penetrating direction, in which a wide area of the non-transfer portion is difficult to obtain, and thus a volume of the shrinkage of the non-transfer portion due to natural shrinkage is insufficient to prevent the sink from appearing on the optical surface. As a result, the sink is generated undesirably on the optical surface. To address this difficulty, blowing the compressed air onto the resin works well to generate deliberately the sink on the non-transfer portion.
Although advantageous and generally effective for its intended purpose, if the integral double-layer plastic lens has the non-transfer portion on the non-optical surface, an absolute value and a gradient of birefringence of the lens near the non-transfer portion are relatively small, but the absolute value and the gradient of birefringence far from the non-transfer portion are large.
In such a case, a wavefront of the transmitted light varies significantly far from the non-transfer portion, causing degradation of the beam waist diameter, which results in degradation of imaging quality.